The submitted form ended up in the spam folder.

Hello, I'm experiencing a strange issue with a contact form. Whenever I fill it out myself for testing purposes, the emails don't end up in the spam folder. However, when other people fill out the form, the emails always end up in spam.

What could be causing this?

Here's the website: https://www.hochzeit-opr.de/

Please click the "Register" button.

Thanks for your help.